Group delay adjuster circuits play a key role in a number of applications which require compensation for group delay, such as feedforward power amplifiers, and the like. In an exemplary feed forward amplifier, a group delay adjusting circuit might be used in loop compensation circuitry such as a phase and gain adjuster. Alternatively, the group delay adjusting circuit may be implemented as a separate functional block prior to the phase and gain adjusting circuit.
In an electrical network transmission without distortion is typically achieved when an amplitude and a group delay response of a network are as close to constant as possible. If an electrical network has a non-constant group delay, group delay compensation in the form of an electric compensation, or group delay adjusting, circuit may be added so that the overall electrical network response is more nearly constant.
As will be appreciated by those skilled in the art the frequency response of a network may be represented as a function of a frequency T, by F(jT)=A(jT)+B(jT) or equivalently as F(jT)=*F(jT)*ejN(T). The magnitude of the amplitude response of the network is defined as *F(jT)*=[A2(jT)+B2(jT)]1/2. The phase angle of the network is defined as N(T)=tan−1 [A(jT)/B(jT)]. The group delay of the network is defined as θ(T)=dN(T)/dT. Two signals having an equal propagation delay have equal phase verses frequency slopes. Two signals have constant delay when the phase verses frequency slope, or group delay is constant. The group delay response of a the network is the response that is sought to be compensated for with a group delay adjusting circuit so that an overall group delay response for the composite response of the two networks tends to be flattened, and thus compensated for. Feed forward power amplifiers amplify multiple carriers, or groups of frequencies.
Often it is desirable to match the characteristics of signal paths to optimize electrical performance. For a feedforward amplifier to be effective over a wide bandwidth in canceling distortion it is desirable to have cancellation loops with the greatest cancellation possible over the greatest bandwidth possible. For example, in a linear feed forward power amplifier (FFPA), one or more error correcting, or error cancellation, loops are present. In each loop a signal will typically travel through an active signal path present in active circuits and a passive signal path through the passive circuits. Loop cancellation tends to be optimal when signals traveling over the active and passive signal paths tend to have equal amplitude responses, opposite phase responses, and equal group delay.
Compensation is typically provided by a network having inductors and capacitors disposed in it to achieve a compensating response. Inductors are usually difficult to build, and often require trimming and/or adjustment. They are typically implemented for example, by a coil of wire wound on a form, or a spiral of foil disposed upon a substrate or printed circuit board.